Hmmm that article Alan linked to seemed to describe exactly my problem, and explained some possible workarounds, but without going into any kind of detail about such workarounds.
One idea -- although I don't exactly understand it -- was to remove the SYSDBA user's role on a database (not that I even know what 'roles' are). But like I say, it had no details so I would have no clue how to even try this.

Thanks for all your help

--- In firebird-support@yahoogroups.com, Dimitry Sibiryakov <sd@...> wrote:
>
> > If I could use embedded firebird for the local database, and deploy a security2.fdb file, would that then mean they could not access the local DB without the correct password?
> > Or could they simply copy the DB file to another PC and access it without a password?
>
> Once again, slowly: if someone has physical access to database file,
> (s)he has full access to data inside it. That's all. No workarounds, no
> way to protect anything. Finish.
>
> SY, SD.
>